Conventionally, exhaust systems of various engine types including automobile engines have been provided with a catalytic converter for cleaning exhaust gases. In general, a catalytic converter basically includes a columnar catalyst, a mat to be wrapped around the catalyst, and an outer cylindrical housing for accommodating the catalyst around which the mat has been wrapped, and by reducing the diameter (swaging) of the outer cylindrical housing the mat is pressed so that the catalyst is supported in the outer cylindrical housing.
The mat to be wrapped around the catalyst is made of heat resisting fibrous material and interposed between the catalyst and the outer cylindrical housing, so that the mat provides various functions such as to elastically support the catalyst and to thermally insulate the catalyst from the outer cylindrical housing as well as functions to ensure the sealing performance for preventing uncleaned exhaust gases from passing through the space between the catalyst and the outer cylindrical housing. Especially, in terms of retaining the capacity of the catalytic converter over an extensive period of time, the mat plays a very important role.
In order for the mat to perform its capacity, it is desirable to provide mats having an identical surface pressure. In particular, in catalysts which improve the exhaust efficiency or the cleaning capacity, walls which form a catalyst are thin walls and thus it is necessary to provide the mats having an identical surface pressure for the purpose of preventing drawbacks such as breakage due to elevated heat or vibrations during the use.
However, since catalysts differ in outer diameter size among the respective products, if the outer cylindrical housings are processed to have the same diameter by the swaging process, it is impossible for the mats to have the identical surface pressure. For this reason, as a general production method for providing the identical surface pressure of the mats, the conventional method comprises measuring an outer diameter of a catalyst, determining a predetermined target amount for reducing the diameter based on the measured value, and swaging the outer cylindrical housing based on the thus determined diameter reduction amount.
As other known methods such as, for example, disclosed by Japanese Laid-open Patent Application No. 2003-343255 (paragraphs 0019-0023; FIGS. 2 and 3), the surface pressure of the mat which has been wrapped around the catalyst is detected by a sensor, and thereafter, measuring the distance between the center of axis of the catalyst and the sensor when the surface pressure takes a predetermined value, and adjusting the diameter reduction at the subsequent swaging process such that the distance from the center of axis of the catalyst to the outer cylindrical housing is equal to this measured distance.